The present invention relates to the artificial illumination arts. It finds particular application in protecting lamp ballasts from open circuit load faults and will be described with particular reference thereto. It is to be appreciated, however, that the present invention is also applicable to protecting vital components of other circuits from voltage and current surges, and is not limited to the aforementioned application.
Often, it is desirable to operate fluorescent lamps independently of each other. In many existing systems, if one lamp fails, others will also cease to function. To aid in the re-lamping process, it is desirable to have all functioning lamps operating, so that a repairman can easily discern which lamps need to be replaced. Also, it is easier for the repairman to work if he has light from adjacent lamps to aid him.
When a lamp fails, or is outright removed from the circuit while the ballast is in operation, open circuit voltages can become so high as to damage sensitive circuit components, rendering the ballast useless for further lighting applications. However in order to replace the lamp while other lamps are in operation, it is necessary to remove the lamp while power is being supplied to the ballast.
Often, ballast circuits include a plurality of transistors, such as bi-polar junction transistors, (BJTs), for switching purposes. In these applications, transistors may be stressed beyond their rated values. In existing ballasts, an open circuit load fault causes greatly increased current flow. With such current flowing through the BJTs, the maximum rated current may be exceeded, resulting in failure of the transistors.
In accordance with one aspect of the present invention, a lighting system powered by an alternating current source is provided. A power factor correction circuit creates a direct current bus, which is used to power a lamp ballast. The ballast includes an alternating current switch that protects circuit components when the lamp is removed or fails to ignite.
In accordance with another aspect of the present invention, a method of protecting a DC lighting ballast from high open circuit voltages is provided. In response to a load being removed from the circuit, an alternating current switch is activated that raises the operating frequency of the ballast, thereby lowering voltage and current flow through the circuit.
In accordance with another aspect of the present invention, an alternating current switch for use in conjunction with a lamp ballast is provided. First and second field effect transistors draw current away from the phase shifting inductor of the ballast. An inductive tap provides a signal to control the gates of the transistors. First and second charge pumps store charge from the inductive tap.